Sophisticated modern cameras typically employ a through-the-lens metering system which senses the amount of light reaching a photosensitive device, for example, a photodiode. By means of an associated electrical circuit, the ambient light reaching the photosensitive device is converted into a signal indicating to the photographer either an overexposed, underexposed or proper metering condition.
Conventionally, the photographer manually adjusts the aperture and shutter speed settings of the camera in response to the exposure signals provided by the meter, usually with the goal of achieving a proper metering condition--i.e., one that the meter indicates is neither underexposed nor overexposed. Frequently, however, it is desirable to select only the aperture of the exposure and to have automatically selected the corresponding shutter speed suitable for a proper exposure. While certain prior art camera metering systems have incorporated this facility, many cameras using through-the-lens metering systems have no provision for such automatic selection of shutter speed. Before the present invention, a photographer with such a manually adjusted camera has been unable to obtain automatic exposure adjustment.
Therefore, a need has existed for a device which may be attached to a conventional camera employing through-the-lens metering to provide automatic selection of shutter speed suitable for a proper exposure.